That can be modded (modified by the end user).(adjective)
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Use "moddable" in a sentence
"First, one way to have a massively popular game series is to make it moddable, because the modding community supports fan interest up until the next release comes out, rather than all those people drifting away."
"I was intrigued with this moddable gadget idea ever since I heard about it last year and these additions are interesting to me, but I'm not so sure everyone else is into it."
"Car gadgetry is a growing market and building a technology that is moddable for many different types of cars is a good idea."