Replace characters in mergefield

[macropod]

Well, you did ask about a VB...

Assuming you can do your merge setup so it outputs:

firstname lastname@companyname.co.uk

you could then do a wildcard Find/Replace, where:
Find = (<[A-Za-z0-9]@>) (<[A-Za-z0-9]@>\@[A-Za-z0-9.]{1,})
Replace = \1.\2
to turn all those strings into:

firstname.lastname@companyname.co.uk

Even so, you wouldn't have clickable email addresses.