Scientific Specific Units

[luisphysics]

This is aimed primarily at Tim.

Scientist like to use certain units because they are useful. I'd like to define my own unit and have it registered to the Unit Table menu, and have CONVERT() know about it.
One such unit is Mass-Energy.
I Tried to do this CONVERT(1_(u),1_MeV/c^2)
but I know I can't do that. I would like a unit called _MeV/c^2 where
1_u = 931.50 _MeV/c^2

This is useful when calculating binding energies in Nuclear physics which is what I do.

Not to take a jab, but my hateful Ti-Inspire CAS can handle this. I can define my own units and the unit converter can do the right thing. Which is about the only time I use it.
If there is a way to make such custom units I haven't been able to find it.

An Idea for notation for such a relation would be
_u==931.50_MeV/_c^2

where == doesn't actually need to be used, but it should represent that these units are related in this way.