How to reopen the Join the Stormcloaks quest?

One of the footnotes on Joining the Stormcloaks (UESP) reads:

On the PC, it is possible to use the console to advance through the quest by entering setstage CW01B stage, where stage is the number of the stage you wish to complete. It is not possible to un-complete (i.e. go back) quest stages.

(Emphasis mine.) Looks like you're not a true brother or sister of Skyrim.

However, if you have a save from before using the console (always a good precaution, but not one we always remember), you could try going back and instead of advancing the Stormcloaks questline, advance the main one. You may miss a bit of content, but at least you'll be able to do both.

To skip past talking to Ulfric, use setstage mq302 30. Alternatively, just complete "Joining the Stormcloaks" (and possibly also "The Jagged Crown") so that Ulfric will stop acting like your mission commander and start acting like a leader at a meeting.

Source: http://www.uesp.net/wiki/Skyrim:Season_Unending#Bugs