If you have an idea that you have previously submitted to Brainstorm that you want to attach to a challenge, simply go to the challenge that you want to add your idea to, and click "submit an idea."  (If there is no "submit an idea" button, check the challenge status in the upper right of the screen to make sure that it is still open for submissions. If it's closed, see the note below.)

You will then see the choice to create a new idea, or use an existing one:

Note that you will see in the dropdown only the ideas that you have submitted or that you participate on as a team member.  (If you wish to add an idea to a challenge but are not on that idea team, see the relevant note below.)

A note about closed challenges: If you don't see the "submit an idea" button on the challenge that you want to attach an idea to, then chances are that the challenge has ended.  The status box in the upper right of the page will confirm that.   If you want to attach an idea anyway, then the end date of the challenge must be extended or removed.  Only an admin or a challenge team member can do that (they just click "edit" next to "status" in the upper right).  They can then re-close the challenge (reset the end date) or leave it open as they like.

A note about attaching others' ideas to challenges: If you think someone else's idea is right for a challenge, we suggest that you point that out to them, and encourage them to add it to the relevant challenge.  That way, they'll be more actively involved in the whole challenge process.  In case that's not practical, get an admin to add themselves to the idea team (on the idea page, the admin clicks "add a new team member" under current list of members), and then add the idea to the challenge.  The admin can then remove themselves from the idea team if they feel that is appropriate.