Yes, there are ways to ‘assign samples to GA pads’.
Start with an empty kit in Groove Agent SE.
Drag a sample onto one of the pads. If you want samples from various kits in the Groove Agent libraries’ vstsound archives, use the browser tab of Groove Agent to browse/find and audition them.
You can also use your OS file system browser and drag samples to a pad from there.
In either case, you can drag them to an empty pad of any fresh user kit, or add them to empty pads of existing “Beat Agent” kits.
Acoustic Agent kits…you can’t modify the pads on those in the same way as Beat Agent kits, nor drag stuff into empty pads, as those kits are more ‘locked down’. You won’t find their individual samples listed in the browser tab like you can for the beat agent kits. If you want samples from those kits you’ll need to ‘resample’ them somehow.
Once you’ve built your kit you can save it in GA if you like.
Trigger the samples with notes on a stave.
For more info making user kits from your own samples I recommend reading the section of the GA SE Manual on “Beat Agent SE Sound Editing”.