If you have the textures, the sounds and the .u packages that some maps are needing from Infiltration, you can play it in UT.
Infiltration install most of the Inf specific (textures/maps/sounds/.u packages/ etc..) in an Infiltration folder (that contains its own maps, sounds, system, textures folders) in the UnrealTournament directory.
And as the majority of the maps you can download for INF will use several files from there, to play the maps in the classic UT, after installation of the Infiltration 2.9 (and of the map pack Desert Storm that contains a lot of additional textures used in some user made maps too) , you have to add the following lines in the
[Core.System] category of your UnrealTournament.ini
Paths=../Infiltration/System/*.u
Paths=../Infiltration/Maps/*.u
Paths=../Infiltration/Maps/*.unr
Paths=../Infiltration/Maps/*.utx
Paths=../Infiltration/Textures/*.utx
Paths=../Infiltration/Textures/Camos/*.utx
Paths=../Infiltration/Sounds/*.uax
Paths=../Infiltration/Music/*.umx
Paths=../Infiltration/Maps/DesertStorm/System/*.u
Paths=../Infiltration/Maps/DesertStorm/Maps/*.u
Paths=../Infiltration/Maps/DesertStorm/Maps/*.unr
Paths=../Infiltration/Maps/DesertStorm/Maps/*.utx
Paths=../Infiltration/Maps/DesertStorm/Textures/*.utx
Paths=../Infiltration/Maps/DesertStorm/Sounds/*.uax
Paths=../Infiltration/Maps/DesertStorm/Music/*.umx
Then you will be able to modify the INF maps into DM (the EAS ones should not be playable in UT without playing them in Infiltration) to play them in UT