yea. the sage modes are all the same thing, they're just expressed differently. you may as well be asking if someone who knows chidori can learn rasengan. they're just manipulations of the same chakra. as i see it, it's not the contracts that give each sage mode its looks, but the methods the user was taught/uses.
Just look at Juugo and how he can form literal cannons by changing the shape of his body. Despite his SM having the same source as his clan, its still distinct. What are scales, pupils, or eye markings to literally forming cannons? Juugo could very well emulate Frog and Snake SM. The real issue is learning the techniques. it'd be like stealing the byakugan but not knowing the gentle fist style. Still P irrelevant though, the only Frog SM tech Naruto himself can use is Frog Kata yet he's considered a perfect user. Mitsuki's another example.