Well, mass can be converted into energy, and therefore into motion, which can put you into a different inertial reference frame, which affects the passage of relative time; I don't know if that's the same as "converting into spacetime", but it does have an effect on spacetime.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort