Groove Agent 4 can import MIDI files and loops. No problem.
It’s should be able to use all the standard unencrypted sample formats such as wav, aif, mp3, flac, etc. So ‘raw sample sets’ won’t be a problem. Note, this won’t bring in all the presets and stuff for a foreign sampler, but if the samples themselves are in a standard format you can use them to build your own GA kits.
If you’re looking at anything that’s bundled with its own sample format for a custom plugin, or uses any sort of encrypted packages…it’s not likely to work in Groove Agent without re-rendering it playing from the native plugin, and building your own GA kits/presets. Cubase makes it pretty easy to ‘instant render’ things if you ever find a need to pull a sample or two from anywhere in your DAW project to drop into Groove Agent for further editing/triggering.
If you happen to also have Halion 5, you may be able to expand the library of ‘raw samples’ that you can manage to extract from a sampler ISO-CD image (I.E. Roland and Akai disks) and convert into something usable in GA. Again, this isn’t going to include kit/preset information for Groove Agent…just the raw samples.
GA does come with a pretty good stash of samples and loops out of the box for the money. The best way to see if it’s a good fit is to get your hands on a Demo and try it out (assuming you already have a Steinberg dongle). You can find the trial download here: http://www.steinberg.net/en/products/vst/groove_agent/trial.html