This is difficult, for multiple reasons.
Proxying FTP is the first difficulty. You’d have to proxy a range of PASV ports, and make sure the backend can exit through the ASA for active FTP traffic, using the same external IP address as incoming FTP control data on port 21.
The second difficulty is the fact that sticky sessions really only work with HTTP (to insert a cookie), so here we would have to use source IP hashing, not roundrobin or leastconn.
Source IP hashing will fail unless both haproxy instance have exactly the same configuration (server and server weight), due to the round robin load balancing on the ASA.
Then, very likely the ASA will get confused with this configuration; I’d suggest to disable FTP connection helpers and configure the incoming ports statically (port 21 + the PASV range of ports).
This is not a simple configuration, you have to understand the dirty little details of FTP very well.