Images rendered from 3D scenes created in an application such as Maya are typically scene-referred linear RGBA images in a floating-point format such as OpenEXR.
When manually specifying color transforms for importing these images for compositing in a scene-linear working space such as ACEScg, you typically need to use one or more of the transforms in the primaries/ directory to convert from the native primaries. For example, if the images use the Rec. 709 primaries, then you can use Rec-709-sRGB_to_CIE-XYZ followed by CIE-XYZ_to_ACES and finally ACES_to_ACEScg.
If the linear CG images have incorrectly had a gamma applied but have not actually been tone-mapped, you can first use one of the transforms in the gamma/ directory to remove the encoding before converting the primaries as described above.
The alpha channel represents the pixel coverage and should remain unmodified by any transform.