I don't think that it can be changed.  I've marked points before using a converter like this http://gps.ordnancesurvey.co.uk/convert.asp

Entering the co-ordinates into the watch is very tedious but gets the desired result.

I think that you can also mark and name points on the watch when you are at the location.

Alternatively if you can pick out the location on the GRG mapping tool you could mark it and save the route to your Garmin.