Afaik, in a mainframe environment, whenever a dataset is to be referred, there is always a mechanism in place which allows a user to tweak the DCB/SPACE parameters.
Couple of thoughts for your query:
a. Have a step in the FTP JCL which defines the dataset in which you want to receive the data; the definition parameters will be governed by JCL rules, hence RLSE will be allowed here.
b. Look at the SEZAINST definitions, you'll be having a member (for eg: EZAFTPAS) which has the default priming parameters defined; though before you do this - highly recommend that you check with your site admin, as these parameters are usually customized site-to-site; and some people might not feel happy with you tweaking a parameter in this PDS.
Hope this helps.