Is it possible to enable Bitlocker on task sequence for Custom Server 2008R2 Wim?

I created a custom Server 2008 R2 image, updated, put a folder on C: containing scripts & powershell scripts for certain tasks.

I've created an MDT Task sequence, but when I run it I do not get prompted about Bitlocker.

It appears to be because its a custom wim - has anyone been able to do this or is it not possible?

For the record it still works with the standard 2008R2 operating systems when you import them from CD.

It also works with my custom Windows 7 images.

thanks

Andrew