Making a user's main group their subgroup in GroupManager without making myself a separate plugin?

I have donations running through Buycraft, using a "Donator" group. I also have a separate "Trusted" group for access to non-grief-protected worlds. I cannot trust all donators, so I cannot inherit trusted from donator. However, I want my Trusted members to retain their Trusted access, but also gain Donator perks, and also have Trusted as their main group after their Donatorship expires. How should I go about doing this?

I can, of course, manually add Trusted as a subgroup of those people, but I want a better solution that will also work when I'm unable to access my server. Here is what I have thought of:

1. Adding another group for DonatorTrusted. I don't know how I will implement this, though, as I can only explicitly refer to one group in /manuadd <player> <group>. This may be possible with promotion/demotion, but I haven't touched that in GM (yet).

2. Adding Trusted as a subgroup when someone donates and making Donator their main group. Here, I cannot figure out how to check if the person is already trusted or not in a command. (Unlike methods, commands do not return anything)

Of course, I could create my own plugin to hook into GroupManager and do this for me, but I'd rather not. Preferably, I want to do this without installing more plugins.