I'm a bit confused, sorry.
If you use the SLA battery only for discharging and not for charging, what do you do when the SLA battery is at capacity?
How will you discharge the SLA to "make more room" for additional LiPo discharging?
If you only want to discharge batteries down to storage and don't want to use the charger for that, I would suggest a small inexpensive discharger, like this...
https://www.banggood.com/AOK-3-in-1-...p-1051214.html
(available on eBay as well, in addition to other websites)
Very detailed thread on this unit over here
https://www.rcgroups.com/forums/show...iPo-discharger
There is also a bit nicer, but more expensive unit here as well
Perfect Regulators - Universal Lipo Discharger Pro V2