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.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac27