I am filing a patent in the united states and I don't

In the US, one can put more than one invention in a patent APPLICATION but only one invention per PATENT is permitted. The US government feels cheated out of filing fees and maintenance fees if they allow more than one invention per patent. But they do not care about more than one invention per APPLICATION - in fact, the government recommends it (more money for them - in the long run).
The benefit to the invention is that he/she can (1) spread out the filing fee costs over years, and (2) keep the patent "family" alive for many years allowing new applications and associated claims to be file 5, 10 even 15 years after the filing of the original application. Number (2) is too complicated to go into here.
If a US patent application discloses inventions A, B, and C, for example, typically the claims are drafted to cover inventions A, B, and C.
When the application is first reviewed (say 1 to 3 years after the application is filed), the Examiner will likely note that the claims cover three inventions and issue a "restriction requirement" which requests the inventor to select one of the three inventions.
Suppose the inventor elects invention A and the claims are amended to ONLY claim invention A and any patent that issues will only cover invention A.
The process of "electing invention A" is called making an "election".
If, after electing invention "A", the inventor wish to have a patent on invention B or C the inventor must file a second application covering one of B and C. Such an application is called a "Division" (if not new matter) or a "Continuation in Part" (if new master is add at the time of filing the second application).
Such a process is recommended by the USPTO.
In the US the claims can be amended to claim any invention disclosed up until the point of making an election. Once an "election" is make only claims and claim amendments directed to the election invention can be submitted.
YOUR QUESTION
"Is it possible to get new claims added in?"
ANSWER
Since you are talking about a "divisions" you are talking about a US application. Notably, Divisional and Continuation in Part applications are not allowed outside the US.
But in the US, if no election has been made, one can add as many claims as one wishes as long as no NEW MATTER is added. Once an application is filed NO NEW MATTER can be added but claims can always be amended AS LONG AS no new matter is added.
Once an election is made (say invention "A") then claim amendments can still be added but with two restricts: (1) NO new matter, and (2) all claims must be directed to the elected invention (e.g. "A").
If you have an additional questions, please ask.