You should definitely be able to introduce it yourself by following the model of other modifications. Our default set of modifications comes from Unimod, but it has been several years since we updated our Unimod XML file. Are you able to find this modification on Unimod? If so, it might be time for us to make an update.
Thanks for the feedback.
--Brendan
P.S. - The current version is v.4.2.0.19009. We recommend you upgrade, though it won't fix this issue.