It sounds like you're talking about OAuth 1.0. Based on that assumption...
Just come up with an access token and token secret yourself, store it in your OAuth 1.0 service provider's token database, and give it to your OAuth 1.0 consumer. When the call to service provider comes in, DNOA will validate the token against your access token database and it will show up as valid.
In other words, if you want a non-standard way of issuing OAuth 1.0 access tokens, just do it yourself -- not need to use DotNetOpenAuth for generating the access token as it is just a series of random characters stored in a database table.