I was reading those Cable & Deadpool issues where Deadpool briefly "joined" the X-Men, it was great. Do you think maybe some day Marvel will decide to cancel all of his unnecessary books and make him an X-Man?

And not rake in all the cash he generates in his solo comics? You wish. 
 
Although, I would like to see it too. :D

" Wasn't he a kinda member in the X-Men arc from his current series? "

I'd hate to see Deadpool as a true member of the X-Men.

No

He's not really a team player. Even when he was with LL&L he did his own thing. He is working with Black Panther at the moment, sure, but only because he's paid to, and he's still out on his own (for now). Unless the X-Men payed him a check monthly, I doubt he'd stick around with them

From what it looks like in classic vol 1, Deadpool just wants to get by and eat, doesn't matter if there ain't no paycheck.

I know is highly umprobable it would happen, I was just wondering how it would be in case it happens.

If it does happen :  
-expect a mercenary in underoos eating mini rice cakes and other snacks in front of the flat screen.   
-expect Deadpool to sneak out of training with his teleportation device.   
-expect all the women in the team to pity him ; on that note,  expect Jean Grey or Emma Frost to peak into his mind and call him a poor tortured soul but not before they call him a pervert.  
-expect Emma Frost to say marriage is compromise to Wolverine. 
-expect Deadpool to trick Cyclops into using his X-Beams just for the hell of it. 
-expect Kitty to get Ryu'ed 
-expect him to drive both Logan's jeep and Cyclops' motorbike 
-expect random people showing up at the mansion looking for a "Deadpool" or a "Wade, Jack Wilson" with a death warrant, but not before they take someone hostage. 
-expect Professor Xavier to snoop around in Deadpool's mind, only to call him a poor tortured soul, but strictly during "therapy" sessions. 
-expect to see a few limbs laying around the mansion from time to time, some underneath the sofa cushions, some behind the fridge, some at the door of the women's bedrooms, some in their closets. 
-expect Deadpool to be doing a garage sale, selling Cyclops' stuff. 
-expect Deadpool to sing in the shower and lose track of time when everyone is trying to get ready in the morning. 
-expect the unexpected. 
-expect Deadpool to make Cyclops impressions. 
-when out on a mission, expect Deadpool to be the first one to remark anything suspicious, and everyone to call bull except Beast. 
-expect Wolverine and Deadpool to have grunt matches, then when Cyclops butts in, they both grunt at him and tell him to shut up. 
-expect Deadpool to make fun of Northstar. 
-expect Deadpool to use his illusion projector and try to get with the ladies.  
-expect Deadpool to taunt the prisoners into fighting him. 
-expect him to be invited to the Brotherhood. 
-expect him to go all the way with Rogue, and still be barely raring to go for a second time. 
 **Cough** Expect the Siryn to go off. :)

" @Donnieman v5.1:  Yeah me too, and just think if deadpool join the X-Men what would wolverine do.... he just hate DP so maybe they wont get along with each other "

It could happen, ya never know.  They've had some pretty unlikely X-Men over the years: Juggernaut, Magneto, Namor, Mystique, Sabertooth, White Queen (yes it was unlikely at one point).

He was a member of the X-FORCE which makes him an x-man, xavier's man.

He tried and it didn't work

i think he will be.