No, a Bitlocker partition cannot be shrunk.
You can see this in Disk Management : Right-click the partition and you will see that
the "Shrink Volume..." option is gray and disabled.
There is no other option than turning encryption off (waiting as required),
then shrinking the partition and creating the second partition in the new
Do NOT try to shrink the partition using any third-party product.
Be sure to backup first your entire disk from inside Windows, in addition to the
backup of your data.
AOMEI Backupper Freeware
is a good backup utility that also has a boot CD/USB for emergencies.