Had almost the same problem, first tried to add functx-library but saw it creates absolute path in the generated xslt2-code, which isn't very good.
Well, turns out you can implement that function, but first you have to do some modifications...
Find your Mapforce installation directory, and MapForceLibraries -subdirectory. From that open the "core.mff", and find
<group name="math functions">
<component name="add" growable="true" growablebasename="value">
<sources>
<datapoint name="value1" type="xs:decimal"/>
<datapoint name="value2" type="xs:decimal"/>
</sources>
<targets>
<datapoint name="result" type="xs:decimal"/>
</targets>
As you can seem the "sources" and "targets" elements seems to define the in- and out data types. As it is, they have only implemented "add"-function for "xs:decimal". You can copy/paste this component, then rename it and give new in- out- data types, in your case they are both "xs:dayTimeDuration". Note that there are implementations for each supported language, but you can omit those that are not needed. Here's
what should work:
<component name="addDayTimeDuration" growable="true" growablebasename="value">
<sources>
<datapoint name="value1" type="xs:dayTimeDuration"/>
<datapoint name="value2" type="xs:dayTimeDuration"/>
</sources>
<targets>
<datapoint name="result" type="xs:dayTimeDuration"/>
</targets>
<implementations>
<implementation language="xslt">
<operator value="+"/>
</implementation>
<implementation language="xslt2">
<operator value="+"/>
</implementation>
<implementation language="builtin">
<function name="Core_Add"/>
</implementation>
</implementations>
<description>
<short>result = value1 + value2</short>
<long>Result is the dayTimeDuration value of adding value1 and value2.</long>
</description>
</component>
Your new function should now appear in the "math functions" and should be good to use.