In STEP7 v11 you can select a DB, right-click, and chose "copy as text".
Then, in STEP7 v5.5, you can open an STL source file and paste the copied text. You then have a source file in v5.5 of the DB from v11. Generate the DB by compiling the source file.
Now you can link the OPC server symbolically to the DB.
As a sidenote, it appears that in v11 you can only do this for DBs and SCL code, whereas in v5.5 you can always generate the STL source for DBs as well as for all OBs, FCs and FBs.