How do I paste the contents of one field into part of another field?

How do I paste the contents of one field into part of another field?

Hello - In my database I have a field called "subdomain", and then I have six other fields called "URL Europe", "URL Alaska", URL Caribbean", etc.

The six fields have exactly the same content for every record, with the exception of the subdomain.  i.e. -
http://[subdomain].europe.myvacation.com
http://[subdomain].alaska.myvacation.com
http://[subdomain].caribbean.myvacation.com

I would like these fields to auto populate once the subdomain has been entered.

What is the code to program this?

Also, as an FYI, I import all my records from Excel.  So perhaps something needs to happen on that end. 

Finally, I already have 260 records that need 4 of these 6 fields added.  Is there a way to go back and autofill records with a command?

Any thoughts or solutions will be much appreciated.

Jon