Hi,
Thank you for reporting this issue. There is a bug in sbmlexport, it should be able to export your custom unit. We are looking into a fix. As a workaround I suggest to define your custom unit uM in terms of micromole/liter:
% Remove the old unit
uM = sbioselect(sbioroot, "Name", "uM");
sbioremovefromlibrary(uM);
% Add the new unit
customUnit = sbiounit("uM", "micromole/liter");
sbioaddtolibrary(customUnit);
Best regards,
Florian
