Socket Enchant or Slot Addition is a system which lets you add slots to mostly old existing gears.
There are two NPCs that will perform Socket Enchant. Depending on the item you wish to socket enchant, you will have to look for the locations of Seiyablem or Leablem.
To start any Socket Enchant (also referred as Slot Addition) process, you must bring the required items and zeny to the corresponding NPC that will do the job for you. Not all items are socket enchantable, only those listed by Seiyablem or Leablem are possible. Please refer to the weapon and armor lists below for the requirements and which NPC to go to.

There are four sets of armors and weapons that are socket enchantable. Each set is categorized into letter-classes C, B, A, and S. Items of different letter-classes will have different success rates in the socket enchanting process.
The general success % ranges for each C, B, A and S classes weapon/armor are listed below. Items in the same class can have different sucess rates. It is best to refer to the success rate for each item individually:

• Class C: 25%
• Class B: 20-25%
• Class A: 10-25%
• Class S: 5-10%
• Special Service: 90% Chance (Offered by Leablem if you ask him about 'the thing he skipped'. Scroll down to the end of this page to find out!)

As noted above, as the value of the item increases the socket enchant chance decreases. The zeny required the slot addition also increases as the difficulty increases.

The chance is the same no matter which town's NPC you went to.

Just in case you didn't read the NPC's warning and dive right into the process. Here are some important notes when dealing with the NPC:

• There is a chance that the process will fail. If the attempt fails, then the weapon and any cards compounded to it will be destroyed.
• The material and zeny required is non-refundable even if the attempt fails.
• Make sure that you don't have multiples of the “to be socket enchanted item” in your inventory. For example, if you have a +7 Manteau and a normal Manteau in your inventory, the NPC will randomly pick on of them for the Slot Addition attempt. So, just remember to only carry one of the equipment to which you want to add a Slot to.