OK, that's good information. I assume your new MacBook Pro has a Thunderbolt port, and I am just about certain your iMac has at least one Firewire 800 port. With that being the case, here is what you can do:
1. Make a backup of everything on your MacBook Pro to an external device (Time Machine is OK, but a bootable backup is better).
2. Shut down the MacBook Pro.
3. Purchase a Thunderbolt-to-Firewire 800 adapter (costs $49 US from Apple).
4. Plug in a Firewire 800 cable to the "Firewire" side of the adapter.
5. Boot up your MacBook pro, and then plug in the adapter to the Thunderbolt port.
6. While the iMac is still off, plug in the Firewire 800 cable to one of the Firewire 800 ports.
7. Boot up your iMac in Target Disk Mode. To do that, hold down the T key while the iMac boots up.
Your iMac's internal drive will appear as a hard disk on your MacBook Pro's desktop. You can then launch Migration Assistant from the MacBook Pro (located inside the Utilities folder within the Applications folder), and then "migrate" (copy) all non-system stuff from the iMac to your MacBook Pro.
Again, the "risK you are taking is that possibly some of your applications will not work under Yosemite, OS 10.10.4, and could possibly be upgraded.
By the way, a new version of Yosemite, OS 10.10.5, was released earlier today. I am waiting until the OS 10.10.5 Combo Updater is available (most likely no later than tomorrow).
By the way, I use such an adapter to connect an external drive, via Firewire 800, to my 13" MacBook Air. It works perfectly!