If I become an imperial legion member can I still join the stormcloaks?
If you want to experience all the quests for the Stormcloaks and the Imperial Legion, you need to do a second playthrough. While you have the opportunity to switch allegiances at a few points, that will only affect which set of quests you do afterwards.
However, the five Guild storylines (Companions, Thief Guild, Mage College, Bards College and Dark Brotherhood) are all independent from each other and from the Civil War storyline. You can do all five with the same character if you want to.