Is there a way to edit character roster?
Fanmade dlc Story Pack adds few charscters to the roster. So there is.
And
Is it posssible to delete some cells or change its position? Change default character order?
Like this:

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There is a file called UI_PS_CHARGRID.xxx
You can open it in hex then you will find these:
cell_subzero subzero cell_scorpion scorpion cell_mileena mileena........etc
cell_subzero is the pictures, and then subzero is the slot. This is how you change the order.
The cell order is from Top left to Bottom right. If you want to swap locations of Subzero and Ermac, then you will have to replace both cell_subzero subzero with cell_ermac ermac, and then search for cell_ermac ermac and replace it with cell_subzero subzero. But becareful, this might not work because "ermac" is smaller than "subzero". So what shall you do in this case? There is a location in the file where it has these:
DLC0 DLC1 Cell0 Cell1 Cell2 Cell3 Cell4 Cell5 Cell6 Cell7 RANDOM Cell8
Between DLC0/DLC1 you will find a bunch of random junk, if you edit those you can move the boxes around. I don't know how exactly, but this is what I do:
Let's assume Ermac is Cell 5 and Subzero is Cell 8, then I would go to Cell 5 and copy it's junk data and then put them for Subzero, and copy Subzero's data for Ermac. This way I change the order.

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IMPORTANT:
Under the US Copyright Law, I am given full ownership of anything I create or do, hence I own ownership of my mods. I hereby give these mods for FREE to anyone who is willing to download them for any purpose under the condition that they do not try to sell them or gain benefits in any way, and if they're going to upload a video or showcase them, they MUST provide a download link to this mod (hence the term Free). Credit not necessary but don't claim as your own or somebody else's.

Is it possible to add third line to the roster?

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Yes you can add a third line, but you will have to remove from the 1st and 2nd lines. It is the same way of the method I discussed around the junk data.

_________________
IMPORTANT:
Under the US Copyright Law, I am given full ownership of anything I create or do, hence I own ownership of my mods. I hereby give these mods for FREE to anyone who is willing to download them for any purpose under the condition that they do not try to sell them or gain benefits in any way, and if they're going to upload a video or showcase them, they MUST provide a download link to this mod (hence the term Free). Credit not necessary but don't claim as your own or somebody else's.

I was only able to change the position of the DLC characters, and of the main characters only change the icons