You can create these using:
Along-track offset distance - IDENT/(+-)DIST[/NAME] - offset plus or minus DIST from IDENT which is currently in the flight plan
Place Bearing Distance - IDENTBRG/DIST[/NAME] - point defined by a distance along a radial from a place
Place Bearing / Place Bearing - IDENTBRG/IDENTBRG[/NAME] - point defined by the intersection of two radials from two places
LAT/LON coordinates. This can be done with Degrees Decimal Minutes, or shorthand as per the usual shorthand rules.
There is some way you can do it using the joystick on the CCP and selecting the point on the MFD map, but I'm not entirely sure how you go about doing that.
The Places in question do not have to be VORs, they can be any arbitrary point, including previously defined custom waypoints - I've created downwind, base, and final legs for visual approaches with VNAV planning using this feature.
For your example specifically I'd use place bearing/distance. As far as I remember, just enter this in the scratchpad:
IDENTBRG/DIST[/NAME]
So in your example, if the VOR was ATY, you'd enter "ATY225/50" which would give you a waypoint 50nm southwest of ATY. If you added an extra trailing slash you could input a name for that waypoint as well.
Then, once that's in the scratchpad, press the LSK next to where you want to insert it, and then add your altitude restriction on the right side as normal!