Is it usually allowed to use grant money for voluntary contributions?
I have to guess on this one, but can imagine that they haven't mentioned it because they haven't thought of it. I also imagine that they would be somewhat upset to learn of it unless you make provision for it in the grant application.
The reason for my speculation is that few grantors are so rich that they don't much care how their, perhaps, limited funds are spent.
But if you use something that is explicitly funded by donations, then you may be able to make a case that you should donate. Your suggestion of Patreon's involvement suggest that you may be in this situation.
But not all &quot;donations&quot; are prohibited or discouraged. Putting grant funded software in public repositories and making it open source or giving generous licenses to it is often encouraged. Another possible form of donation is the funding of a person so that they can donate their time and efforts to worthy public projects that can't afford their own funding.
I would ask the grant administrator before doing this, and suspect that different people will give different answers. And if the donations are a minuscule part of the grant, it is less likely to be forbidden. But that, too, is speculation.