From manual of ProFTP:
Proftpd will not create files that have the execution bit turned on, this is a security driven design decision. The permissions of the uploaded file can be changed by issuing a SITE CHMOD command can be used to change the mode of the uploaded file. Syntax of the command is: SITE CHMOD <mode> <file>.
So, it can't. But in Linux you can anything. I see 2 solutions:
1. Recompile ProFTP, that it allow of creating files with execution bit set (somewhere should be 'open' function, just need set proper flags).
2. Use inotify (it can be used even in shell scripts) to automatically change file permission after creating it.