Hoemee, Yes, you will need to clear the foreign first.  So you currently have a 3 drive raid 5 with a drive in foreign and a drive still sitting in as a HotSpare, or it already rebuilt? If so then it looks like you had a failed and a foreign. Is the OS bootable currently?  You should be able to do this all from OMSA, so long as it is a current version. 
Dell PE840, running OMSA 6.5.0......RAID 5 with 3 HDD's and one hot spare.....the failed drive indicates foreign and the hot spare was used to replace the failed drive.  Do I just plug in the new drive and set it to global hot spare?  Do I have to clear the foreign status first?  And, can I do all tasks from within the OMSA?   thanks 
Dell PE840 replacing failed drive