Well yeah of course it is possible there are 2 ways 1 is what i personally call "recycling" (which is my favorite way) and the 2nd you can create a file where all your custom items will be there.
Way 1 "Recycling": well is pretty straight forward you have to look for an item that is no longer used and change its properties for example the item 00001 is a quest item that is not used you can transform it into a weapon.
Way 2: locate in the item folder which is the last number file for example 24000-24099.xml and create a copy of it and name it 24100-24199.xml and in there you can create all of your custom items.
Why do i prefer "recycle" way. Is quite simple actually. The more files a server has the more time it will take to load it up BUT this thing has its downsides of you plan to upgrade you server from interlude let's say to Kamael it will take you more time to locate the custom files.
Lastly whatever way you choose you need to create an item in both Itemname.classic.dat and WeaponGrp.classic.dat
Now if you want to tranfer visuals between classic and main you need to also locate the file Itemname.dat and weaponGrp.dat. Copy the lines of your desired items and paste them on the files Itemname.classic.eu.dat and WeaponGrp.clasis.dat accordingly. REMEMBER!!! 2 things. 1 the id of the item needs to match with the ID of your item in the server files AND 2 don't get confused with the .ClassicAden.dat files those ones are for essence ignore them.
Hope you can do it