Yes, Hamas uses donations that are meant for civilian and humanitarian aid toward military use. We don't have the exact amounts, and we will likely never have, but there are clear evidence that it did so on a regular basis.
I want to start with what is (in my eyes) not only the most direct evidence of it but also is one of the most cynical cases. Hamas dug out water pipes donated by the EU and turned them into rockets. And made a video boasting of it.
EU funded water pipelines despite Hamas boast it could turn them into rockets, the Telegraph (behind a pay wall).
We then have, again, self published photographic evidence of them using humanitarian aid for military purposes. In this video by Ryan McBeth he analyses a photo and shows how bags that were supplied with grain from Japan as aid were turned into fortifications in a Hamas bunker.
We also had a Tweet by Unrwa that Hamas affiliated men took supplies from them, which was later deleted and denied.
The story reported by the NYT and WSJ (behind a pay walls). The NYT article was summarized by the National Review:
The Palestinian terrorist group Hamas has reportedly built up a months-long stockpile of food, fuel, and medical supplies as humanitarian resources for the Gaza Strip have dwindled, the New York Times reports.
However, there appear to be instances within the last several weeks of conflict where Hamas has done just that. A since-deleted statement from the United Nations Relief and Works Agency for Palestine Refugees (UNRWA) argued that Hamas looted a warehouse and stole fuel and medical supplies meant for Gazan refugees last week. The agency shortly after retracted the statement and explained that the disappearance of supplies represented merely “a movement of basic medical supplies from the UNRWA warehouse to health partners,” but did not specify which partners.
“The Hamas movement cares only about the Hamas movement,” an Egyptian analyst told the Times. “The public of Gaza mean absolutely nothing for Hamas.”
Another aspect of taking Humanitarian aid is diverting concrete and other building materials from civilian use. Hamas had 500 km of fighting infrastructure in the form of tunnels under all of the Gaza strip:
For years, the Iranian-backed militant group — which has been classified as a terrorist organization by several countries, including Canada — has been spending millions of dollars fortifying the tunnels that it claims stretch for 500 kilometres.
Cement for rebuilding Gaza diverted to terror tunnels, Times of Israel:
The Hamas terror group has been redoubling its efforts to restore the cross-border offensive tunnels that were destroyed by Israel during last summer’s war in the Gaza Strip, Israeli media reported Friday morning.
According to the reports, some of the cement and other materials being delivered to the coastal Palestinian territory, as part of an international rebuilding effort, has been diverted to the tunnels.
In case there were any doubts as to the purpose of the Hamas tunnel system, In a recent interview to RT in Arabic Mousa Abu Marzouk admitted that the tunnels are meant only for fighters and not for the defence of Gaza's citizens and that Hamas does not take the well being of and safety Gaza's citizens as their responsibility.
Many people are asking, since you have built 500 km of tunnels, why haven't you built bomb shelters, where civilians can hide during bombardments?
We have built the tunnels because we have no other way of protecting ourselves from being targeted and killed. These tunnels are meant to protect us from the airplanes. We are fighting from inside the tunnels. Everybody knows that 75% of the people in the Gaza strip are refugees, and it's the responsibility of the UN to protect them.