Originally posted on 08/02/2015:
AFAIK, no...making you state up front how many "as yet un-used" legs your parlay will contain is, in fact, "what's in it for them"...cuz it takes away your freedom to say uncle and cash out at your convenience AND forces you to roll future dice with them, and not at a competitor on said future legs you are obligated to fill...luckily you always have the option to make the number of "legs to be determined later" equal to ZERO by submitting a plain old parlay
you're right tho'...you could always take a college football/cricket/boxing "squash" match or two when you get antsy...
despite the availability of that "solution" (which could go very wrong--eg. Rousey could have broken an ankle last night as the 15th leg of an open 15-legger...BTW, i love your leaning towards int'l volleyball tourney mismatches in such situations as this introduces MUCH lower variance than almost anything outside of possibly snooker or darts)...anyhow, my approach would always be to specify zero "to be added later" legs...and go with the minimum #legs you need right now, say ABC...once it's cashed and turned your initial 5.00 into 9.73 for example...you can manually add "D...E...F and beyond" legs using a succession of straights and minimally-legged parlays starting from 9.73 at the "D..." point
nothing (apart from record-keeping "cleanliness") is to be gained by locking yourself in to buying a set number of future legs at the same book that you happened to start the first few legs of your growing $-mission at