I have been collecting points for a bit and want to make by first Reward Booking.

I want to book a flight from Buffalo(BUF) or Toronto(YYZ) to Zagreb(ZAG).

YYZ seems to have much better availability and I can easily drive there from Buffalo.

I have around 150,000 United Miles and see the following Coach Flight for 30,000 miles.

Would I call United to book the flight?

Is there anyway to book with a stopover, such as DUB -> YYZ (Stop) YYZ -> ZAG that would make sense point wise and then change the DUB -> YYZ to a later date?

This is the flight that shows up in the Rewards Search:

30,000 Miles and $82.80

Depart:
8:10 p.m.
Fri., Apr. 20, 2012
Toronto, ON Canada (YYZ)
Arrive:
10:05 a.m. +1 Day
Sat., Apr. 21, 2012
Munich, Germany (MUC)
Flight Time:
7 hr 55 mn
Flight: AC846
Operated by Air Canada.
Aircraft: Airbus A330-300
Meal: Meal
No Special Meal Offered.
See On-Time Performance
Change Planes. Connect time in Munich, Germany (MUC) is 1 hour 30 minutes.

Depart:
11:35 a.m.
Sat., Apr. 21, 2012
Munich, Germany (MUC)
Arrive:
12:40 p.m.
Sat., Apr. 21, 2012
Zagreb HR (ZAG)
Flight Time:
1 hr 5 mn

Travel Time:
10 hr 30 mn
Flight: LH1712
Operated by Lufthansa Cityline.
Aircraft: Avro RJ85
Meal: Snack or Brunch
No Special Meal Offered.
See On-Time Performance

Since your questions are specific for United (each program has different rules for stopovers etc) it is better to ask these questions in the United forum.
And yes: you book thru United.

Here is your option, open up a Continental Account, link your UA mileage plus to your CO one pass account, transfer the miles to CO and book it from CO website. Or if you can wait, just wait after 3/3 then you can book it there.

Your 2nd question is not really a stop over, as you can't go back and forth Atlantic ocean. You can book 2 one way reward however. Also, in UACO one-way award don't allow stop over, round trip award allow 1 stop-over or 1 open-jaw.