I was wondering if there is a way to activate the signature when required i.e for selective email addresses or on an ad-hoc basis (using Outlook 2007)

Possibly there are free add-ins that allows one to activate the signature

Your assistance in this regard will be most appreciated

You can have a different signature for individual email accounts in Outlook.
Is your question can you have a different signature for a specific email address in yoor contacts?

Thanks for the reply

I would like to know whether it is possible to have a different signature for a specific email address in my contacts? If so how do Igo about setting this up