Is it possible to create multi-record custom fields on Activities?
Not currently.

In general, feature requests like this can be handled in a few ways:


If you are a developer, you can submit a pull-request with the new feature.
If you have funds to contribute, you can submit a feature-request to the paid issue queue for the core team to work on.
You can work around your problem using existing features. For example you could set up the custom field as a "Note" textarea and put the multiple funding sources each on a separate line of the same field.