Sure you can link from anywhere you want. You just have to make sure that you follow Adsense policies.

Here is what Google says about traffic:

"Google ads may not be placed on pages receiving traffic from certain sources. For example, publishers may not participate in paid-to-click programs, send unwanted emails or display ads as the result of the action of any software application. Also, publishers using online advertising must ensure that their pages comply with Google's Landing Page Quality Guidelines."

Paid traffic is also allowed. Google even allows Adwords traffic. Click on the link above to read what is allowed and what is not.

I hope that this helps.

Tony Alves