The next time you take a check ride in the DO you can just have them issue the ATP at the same time if there is a Designated Examiner in the house. All the items int he 121 or 135 checkride are identical so its just a paperwork formality. My ATP was free when I did my last checkride at Simuflite
Are you currently flying the 328? How about getting your written out of the way, then wait for your upgrade? You spend about $100 on study materials and the computer exam, then the flight test is no cost to you. It's incorporated into your IPC.
Do what the others say. Knock off the written and take the practical as part of your annual or six-month check. Do it on the company's nickel.
Two other suggestions, after you take the written: See if there's an examiner at your local airport and a light twin and do it that way. You are probably current and proficient enough that you would need only a familiarization flight or two in the twin. Maybe some sim or dual with an instrument instructor to brush up.