No o.o
Just, name your custom SPR and DAT files, as normal. Tibia.spr and Tibia.dat, then in RME, go to: File > Preferences > Client Version.
Then, uncheck the option "Check file signatures". And browse to your custom files ;D
Also remember to replace the items.OTB and items.XML files in the directory where RME is installed. Those files are in data/"your version" like 850 or that.
PS. In the editor, when you replaced the items.xml file, some items will not have name IN the editor only. This, because the editor doesn't support "fromid, toid". But in the OT, theres no problem ;D