It's a pretty esoteric request so I don't really see much use for it outside of really specific/niche situations....thus not something I would add as a feature.
One thing you can do as an alternative: Create a 2nd flow, import the first using Birth Flow/Flow Update operators, and use the offset spinners to allow the first flow to be imported at a later frame. So first flow could run frame 0-50, and 2nd flow could import the first and continue from frame 1000 and onward, for example. This wouldn't work for complex setups involving bindings, actors, cloth, etc...but for basic shapes and velocities and such it should work fine.
One thing you can do as an alternative: Create a 2nd flow, import the first using Birth Flow/Flow Update operators, and use the offset spinners to allow the first flow to be imported at a later frame. So first flow could run frame 0-50, and 2nd flow could import the first and continue from frame 1000 and onward, for example. This wouldn't work for complex setups involving bindings, actors, cloth, etc...but for basic shapes and velocities and such it should work fine.