If I remember right, all you have to do is some server-sided scriptwork. I'll see if I can dig up anymore information.
EDIT:
1) open Gamemode_Teamdeathmatch / Gamemode_Team Deathmatch.cs
2) open Gamemode_Siege / Gamemode_Siege.cs
3) copy and paste the "addGameModeRule("Team Deathmatch", "Lives", "int 0 10", 0);" line into Gamemode_Siege, at the top (BUT BELOW "addGameModeRule("Siege", "Time", "int 1 30", 10);")
4) replace "Team Deathmatch" in the copied line with "Siege".
5) Scroll to Gamemode_TDM_onMiniReset in both files
6) Copy and paste the following from Gamemode_TeamDeathmatch into Gamemode_Siege, both above the existing lines in Gamemode_TDM_onMiniReset, but still in the function
if(%mini.tdmModeRule[1] != 0)
%mini.messageAll('',"\c5You have \c3" @ %mini.tdmModeRule[1] @ "\c5 " @ (%mini.tdmModeRule[1] == 1 ? "life" : "lives") @ " remaining.");
and
%mini.setLivesLimit(%mini.tdmModeRule[1]);
That should do it, unless I've overlooked something.