If you want to use CapyTalk, here is a little generic script:
"Loop back and forth within a given range"
| dur start end |
dur := 'nameOfPsiFile' fileDuration.
start := -1.
end := -0.9.
"Convert to Range01"
start := start * 0.5 + 0.5.
end := end * 0.5 + 0.5.
((1 repeatingTriangle: (dur * 2 * (end - start)))
* 2 * (end - start)) + (start * 2 - 1)
It might be even simpler to use the TimeIndexForFilename or TimeIndex classes and paste them into the TimeIndex parameter field of the Morph2DKeymappedPsi.
Hope that helps :)
EDIT: In case you want to use the LFO Prototype you could use the SetRange class:
LFO -> SetRange -> TimeIndex parameter field