As a general rule, spell resistance is 11+CR so you would simply add the additional challenge rating to the creature's spell resistance directly.
From the monster manual page 300 sidebar, BEHIND THE CURTAIN: SPELL RESISTANCE AND DAMAGE REDUCTION:
Spell Resistance: If you choose to give your monster this ability, you’ll probably want to set the resistance number equal to the creature’s CR + 11. This means that a character of a level equal to the creature’s will have a 50% chance to overcome the monster’s spell resistance (barring the Spell Penetration feat). For example, a 12th-level character has a 50% chance to overcome spell resistance 23, so 23 is a good spell resistance number for a CR 12 creature. You may need to adjust a creature’s spell resistance number after you finally settle on a CR for the creature. If you want a highly magic-resistant creature, set the monster’s spell resistance higher than CR + 11. For lesser resistance, set the spell resistance lower. For each point of difference, you’ll change the chance of successfully overcoming spell resistance by 5%. For example, a 12th-level caster has a 45% chance to overcome spell resistance 24, and no chance to overcome spell resistance 33.