Hi Scott,
An APBS OpenDX file is just a text file which lists the values at the grid points (x-axis varying fastest, z axis varying slowest). So you could probably write a script to convert it to this other text format easily enough. A Chimera Python script could do the same thing but it isn't any easier. Still if you wanted to try it using Chimera Python here is a basic example of accessing volume data in Chimera
Below is the start of an APBS OpenDX file to give you an idea of how you would write a script to read it and reformat it how you want.
By the way, we haven't developed Chimera for about 8 years and our new program ChimeraX is much more capable, but would still need a similar Python script to output values in the format you want.
Tom
# Data from APBS 0.3.2
#
# POTENTIAL (kT/e)
#
object 1 class gridpositions counts 65 97 97
origin -8.064450e+00 -4.768105e+01 -4.266980e+01
delta 3.779047e-01 0.000000e+00 0.000000e+00
delta 0.000000e+00 4.098240e-01 0.000000e+00
delta 0.000000e+00 0.000000e+00 4.099125e-01
object 2 class gridconnections counts 65 97 97
object 3 class array type double rank 0 items 611585 data follows
8.921700e-01 9.000006e-01 9.078676e-01
9.157664e-01 9.236924e-01 9.316403e-01
9.396046e-01 9.475795e-01 9.555586e-01
9.635354e-01 9.715027e-01 9.794531e-01
9.873788e-01 9.952714e-01 1.003122e+00
...
(65 * 97 * 97 values in the file).