You could select the source text property and then choose in the Ae menu
Animation->Keyframe Assistant->Convert Expression into Keyframes
It is not ideal, since this will give you a keyframe on each frame, but it should work. With a second click, you can also click on the keyframe stopwatch to delete all keyframes (since all will have the identical text).
You would need to do this for each text layer individually, however.
The tool Easy Bake can convert all expressions of your project into keyframes at once and also minimize the keyframes. Not 100% sure if it supports texts, but it might be worth to try it