If you want to add a new financial dimension to the system, this has become much easier with the release of AX 4.0 which includes a wizard to do so.
The wizard is started from the file menu, under Tools \ Development tools \ Wizards \ Financial Dimension Wizard. It's located under Development tools because it makes changes in the Application Object Tree (AOT) and you need developer access rights to execute it.
The wizard collects the necessary input and performs the needed changes to the AOT.
The enumerated value suggested by the wizard should not be less than what the wizard suggests, leaving a huge range of numbers for dimensions free to be utilized by Microsoft in future versions.
If your application is hooked up to a version control system, the wizard will check out the elements that need to be changed. It will however not check these in automatically.
There is a small bug in the wizard, making it skip a data type (the bug still exists in AX 2009). Here's is how to fix it (and similar bugs if you find any): Missing datatype in the Fincancial Dimension Wizard
I encourage you to read this blog article on how to design your application for more than the three standard dimensions: Write applications that gracefully accepts new financial dimensions
There's is a limit of 10 on the total number of dimensions. This is due to the AccountPeriodIdx on LedgerBalanceDimTrans. You will hit the maximum number of fields that must be in a primary index on Microsoft SQL Server of you add more dimensions. You could discuss the value if having the index if you have that many dimensions and you could consider changing the index to a subset of the dimensions if you only need to do your reporting on some of them. If you change the index, you are not bound of the 10 dimensions limit.