Concatenation is your only option there. If you want to create a striped metaLUN the LUNs have to be the same size, RAID type, and disk type.
I am trying to create a LUN using the free space in two separate RAID 5 RAID groups.  I created a 1391GB LUN from the free space in raid group 1 and a 586GB LUN from the free space in raid group 0.  When I try to expand the 1391GB LUN, the only time I see the available LUN to add is when I choose the concatenation option--the free LUN is not listed when I choose striping.  It has been a while since I set this system up, so I am probably just missing something.  Any suggestions?  Here are system particulars in case they are needed:  system:  CX3-40f flare version:  6.26 drive types:  300GB FC Server:  Windows 2003, SP2
Problem expanding LUN