Hello Julien,
I'm not really sure if I'm tackling the issue here but you can always write more lines to (I believe) any keyword that needs a bunch of parameters in your steering file as long as you start them with a semicolon and no spaces inbetween lines of the same keyword and don't exceed the predefined max of those parameters.
As an example I once had a bunch of sources which made me format the steering file
exactly this way:
ABSCISSAE OF SOURCES =825106.750;835321.125;849660.375
;854766.125;861162.4375;864480.625;858905.125;858597.375
;866351.125;871136.1875;876379.0625;891099.250;892734.625
;897153.9375;903053.750;923386.750
ORDINATES OF SOURCES =1281397.125;1288179.375
;1293914.500;1298007.375;1304469.750;1314443.375;1327975.625
;1330903.625;1338208.625;1347119.250;1372284.500;1378749.500
;1377705.625;1379261.375;1380006.125;1387408.500
WATER DISCHARGE OF SOURCES =38.15;17.27;20.56;21.71;19.65
;14.82;18.39;73.35;13.70;11.32;44.38
;-230.94;-370.56;-130.84;-295.04;1626.30
Note that in this case the characters per line in the stereering case file didn't exceed the limit and for the 'sources' definition I stayed under the hardcoded max value predefined in the subroutine...
Hope it helps somehow,
José Díaz.