Well, not out-of-the-box, but you can do:

```
>>> u.def_unit('kms', u.km / u.s, register=True)
Unit("kms")
>>> x = 1.0 * (u.mile / u.hr)
>>> x.kms
0.00044704
```

In other words, it works with any unit in the registry, whether built-in or not.