I haven't done it myself, but I believe you can set a "trigger" attribute. This can be done either per sample, or at the <group> level if you group your samples by type, i.e. Soft, Loud, Release Triggers. There are, of course, other ways to use groups.
The valid values for this "trigger" parameter are, to quote the documentation:
Valid values: attack means a sample is played when the note on message is received. release means the sample is played when the note off message is received (aka a release trigger). first means that the sample will only be played if no other notes are playing. legato means that the sample will only be played if some other notes are already playing. This can also be set at the <group> level. Default: attack.
You'll also want the 'pitchKeyTrack="0"' parameter for the release samples, which makes it so that the pitch will stay the same regardless of what note is played.
So I think your groups element might look something like this:
<groups>
<group tags="NotesSoft_Piano" seqMode="round_robin">
<sample loNote="0" hiNote="53" rootNote="53" loVel="0" hiVel="89" seqPosition="1" path="Samples/NOTE_p_53_RR01.wav"/>
<sample loNote="0" hiNote="53" rootNote="53" loVel="0" hiVel="89" seqPosition="2" path="Samples/NOTE_p_53_RR02.wav"/>
<sample loNote="54" hiNote="127" rootNote="81" loVel="0" hiVel="89" seqPosition="1" path="Samples/NOTE_p_81_RR01.wav"/>
<sample loNote="54" hiNote="127" rootNote="81" loVel="0" hiVel="89" seqPosition="2" path="Samples/NOTE_p_81_RR02.wav"/>
</group>
<group tags="NotesLoud_Forte" seqMode="round_robin">
<sample loNote="0" hiNote="53" rootNote="53" loVel="90" hiVel="127" seqPosition="1" path="Samples/NOTE_f_53_RR01.wav"/>
<sample loNote="0" hiNote="53" rootNote="53" loVel="90" hiVel="127" seqPosition="2" path="Samples/NOTE_f_53_RR02.wav"/>
<sample loNote="54" hiNote="127" rootNote="81" loVel="90" hiVel="127" seqPosition="1" path="Samples/NOTE_f_81_RR01.wav"/>
<sample loNote="54" hiNote="127" rootNote="81" loVel="90" hiVel="127" seqPosition="2" path="Samples/NOTE_f_81_RR02.wav"/>
</group>
<group tags="Key_Up_Noise" pitchKeyTrack="0" trigger="release" seqMode="round_robin">
<sample loNote="0" hiNote="53" rootNote="127" seqPosition="1" path="Samples/RELEASE_SAMPLE_RR01.wav"/>
<sample loNote="0" hiNote="53" rootNote="127" seqPosition="2" path="Samples/RELEASE_SAMPLE_RR02.wav"/>
<sample loNote="0" hiNote="53" rootNote="127" seqPosition="3" path="Samples/RELEASE_SAMPLE_RR03.wav"/>
<sample loNote="0" hiNote="53" rootNote="127" seqPosition="4" path="Samples/RELEASE_SAMPLE_RR04.wav"/>
</group>
</groups>
I hope that's right, and hope it helps!
(And sorry the formatting gets so mangled on here, but you should be able to copy it into a text editor to read it easier if you want.)