You need 2 people to do this, the other one needs to be the party leader

1) Make the party leader join a private match

2) Press on Create a Class

3) Choose a class

4) Press on Copy

5) Make the party leader leave the private match with you so you go back to the party screen

6) You can now scroll though the menu in the background, go to create a class and select a class

7) Press on primary and choose a weapon that has the proficiency you want on another weapon, for example if you want to use speed on another weapon choose a LMG or sniper rifle

8) Select None as the proficiency

9) Select Done on the attachments page

10) Now press back ( O on PS3 ), you can control the copy part from before

11) Select the custom class slot you want your new weapon to be in

12) Go to primary and select the weapon you want the new proficiency to be on

13) Select None as proficiency then when you get the attachments page keep pressing back until you reach the proficiency page from the origional weapon

14) You can now see the new proficiency in the list and its selectable, select it and then you can edit the class as normal after that

Heres a short clip of me adding damage to the mk14 making it a 1 hit kill close and medium range and 2 hit kill at long range