If/Then Triggers and Smart Campaigns

I am trying to set an "if/then" smart campaign.

If lead comes in with this attribute (for my purposes, member of an ad group in AdWords), then I want than I want the data in a different custom field to be automatically updated.

For example: If lead comes in as part of Ad group = 1", then"Source Description" (which is a custom field) will then be filled with = "A".

Although I can set that smart list up going backwards, I want to set it up for all leads going forward so when a lead enters our data base, the new custom field will be populated with the information I need. 

Is this possible?