Currently we do not support this, but using the custom road segments should be quite easy to implement. I have opened an issue for this on StreetArch github, you may place thumbs up on the issue to make it more important to us.
Regarding the custom intersection meshes - the purpose of the StreetArch plugin is to enable creating intersections procedurally. Are you interested in having the possibility to use custom meshes in some intersection in addition to procedural ones? It can be done, but you will lose a lot of features for intersection editing and etc. Do you expect some kind of snapping for this? Please explain this further @gotgrassct so we can add an feature request issue for this too if we can understand it better.