Taken from bleachernation:
"But what about prospects who weren’t on the 40-man until they were called up, say, on September 5? Well, that’s still easy enough: you can also include players on the playoff roster as “injury replacements,” taking the place of ANYONE on the 60-day IL, as long as that player has been out his 60+ days. By the time late-September rolls around, literally every team in baseball history has at least a couple guys on the 60-day IL. So if you want some youngsters on the playoff roster, boom, they “replace” the injured guys.
For the Cubs, there are already FIVE guys on the 60-day IL, several of whom are simply not going to return this year. So the playoff roster thing is – like every year – a non-issue. Were you in the organization as of August 31? Cool. You can be on the playoff roster, no problem."
Tl;Dr : you can designate non 40 man players in the minors as injury replacements and they are thus eligible