Mlppotterwhoovian

Ok, revised entry:
No you shouldn't; its a disgusting combo.

But you probably will anyhow; most whoovians seem to at least once

I can't... they seriously don't sell these in my area  

Depends on where you are geographically I guess. and timeline of course. On earth, In the 20th century, most supermarkets would have frozen fish fingers and packets of instant custard. It is considered normal to cook said fish fingers and indeed, to make up instant custard hot (rather than using cold ready-made custard or confectioner's custard)

I am not entirely sure that doing this is in any way "normal" though, so there's that...