No, if an element existing in ARCHICAD was not created by Grasshopper, then it is not possible to change its length or any of its geometry. If you Deconstructed an ARCHICAD Wall in Grasshopper, changed the length parameters and generated an ARCHICAD Wall from Grasshopper, it would not replace the original ARCHICAD Wall. Instead, it would generate another Wall on top of it.
Ability to modify the geometry or settings/parameters/properties of ARCHICAD elements from Grasshopper is one of the wishes for the ARCHICAD-Grasshopper Connection.
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