I need to su to idmba while in a ksh script run as userx, is there any way to do this? TIA 
su idmba -c "commands to execute as idmba user"
You may well want to add a dash ' - ' in the su statement to ensure that idmba's profile is picked up before command execution.
Would look like ...
su - idmba -c "commands to execute as idmba user"
HTH 
Cheers,
Cameron