Well,
I dont completely understand where .com is. If it is hosted elswhere, not on TCH servers, then you would want to see if you can forward the domain at the registrar level. If the .com IS here at TCH you can open a ticket and have us park it in the .com.
If I read your question like I think it should be then the first option would be the way to go.
Then again if you have hosting space here for the .lu ccount then you could use the redirect feature in cPanel but that is a waste of a hosting account.
Confused? I am.