Install ADT plugin
Right click on java project
Select Android > Convert to Android project.
Second Way: UPD
Actually, the "First Way" doesn't work with latest ADT plugin versions, for some reason google has removed the convert command. Another way would be:
Check in your java project into a VCS (svn or git or whatever)
Delete it from workspace
Import the project from VCS
On import, select Create New Project -> Android
Third Way: UDP :)
Remove the java project from workspace
Start create new Android project wizard
Select create from existing source option and point to your java project folder.