Do you need the collection to be available to the public online?
What other sorts of features do you need?
What other sorts of features do you want?
How large is the collection?
Depending on your needs, your best option might be to put it in your existing database as a loan, or whatever non-accessioned terminology seems fitting.
MusArch is not open source.
If the collection is small, and you’re not insistent on open source, you might look at eHive.
CollectionSpace and CollectiveAccess are the two main open source collections management systems that I’m familiar with. We are in the process of transitioning from PastPerfect to CollectiveAccess, and we like CA more every day. It’s very flexible, customizable, and includes a lot of features we find helpful.