Externship (part-time) while working full time?

I was recently contacted to apply to an externship position (that is 20/hrs a week - part time) at a VC fund; however, I already work full-time. I think the recruiter was aware of this but still sent it to me anyways, and I really like the place (and I think this could be a direct recruiting pipeline).

Has this been done? Is it feasible for me to work a full time job and an externship? My full time job is pretty flexible in terms of time allocation so I can usually shift duties there to the evening. If I can't, how common is it for post-undergrads to take on externships?