My Kudo3D Titan 1 printer was in use with other experiments, so I tested this outside the printer using the UV curing chamber.

EP9009 plasticizer is listed for use with all of Eager Plastics' casting systems, and I found it also to work with MakerJuice G+ resin, making it less brittle and more flexible. It isn't magic though. Since the more plasticizer you put in, the less polymer base there is, there will be a tradeoff. At the 10-20% range, you'll likely see a substantially tougher part since there may be a modest loss of tensile strength in exchange for substantially more elongation. At the 50% I demonstrate in the video, you'll get a substantially more fleixble part, but it will also be weaker. This might get you to a more ABS-like texture, but won't make a flexible resin on its own.

At something like $40-50 a quart, it's cheaper than photopolymer resin.

http://www.eagerplastics.com/plasticizer.htm
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https://www.youtube.com/watch?v=KkLD0lRi404