For custom Drum Parts the name of the game would be replacing kit pieces with samples of similar type.  There are no rules here but the built in arpeggios are customized to particular kits which use up to 16 MIDI notes and each of these MIDI notes is a different drum key.  You can play the drum arp and use a DAW's piano roll to determine which MIDI notes the arp uses then replace each of those note's sample (waveform) with a like waveform.  So say there's a snare sound there - probably best to stick to a snare.  Or crash ... Or bass drum ...

If creating a custom arp then I would still use a mapping of drum keys that matches a built-in kit in order to allow for having that custom kit work both with your custom arp and also preset arps.