Hi,
It will depend what versions of OS you have on your switches but from checking the documentation, I think the commands you need for mapping between EXP and DSCP values are "mpls-exp-inbound" and "mpls-exp-outbound". Both these commands are documented in the MPLS sections of the documentation and both are definitley available in VRP V2R5 on S5700 and S770 switches.
There are also some "mpls-qos" commands which may also provide suitable functionality, I am afraid I have not used any of these commands, so cannot provide any examples or advice but hopefully this will give you a starting point!
Regards Nigel