You want to use custom fields in OLAP cubes in Project Server 2007.

You can can find an in-depth reference which shows what fields and types are supported for cube dimensions and measures in the OLAP Cube for Project Server 2007 at the following MSDN address:
Local and Enterprise Custom Fields

The Project Server Cube Build Service (CBS) summarized rules for including custom fields in OLAP cubes are as follows:

Custom fields in general:

• Cannot be added as measures for the timephased cubes.
• Cannot be added with formulas.

Dimensions:

• Support only custom fields with lookup tables.
• Support Flag custom fields, which cannot have a lookup table.

Task custom fields:

• If the task custom field has a lookup table, it cannot roll up to summary
  tasks and cannot be used as a measure.

Measures:

• Support only Cost, Duration, and Number custom fields for Resource and Project entities without a lookup table.
• If a Cost, Duration, or Number custom field for a Task entity has a lookup table, it cannot be a measure.
• If a Task custom field has a Sum rollup for task summary rows, it cannot be a measure.
• Fields that have an Assignment roll-down and attributes set to None can be dimensions or measures.

Multivalue fields:

• Cannot be included in the cubes because of a limitation in Analysis Services 2000.

Summary tasks:

• The CBS does sum values of lookup tables for the summary tasks. However, if the lookup table does not include a value, Project Professional shows #Error for summary task rows.

Note: Only the non-timephased cubes can contain calculated members.

For example, if you add the custom project field with lookup table to show the project status, you can add it to Cube Dimensions and see it in Analysis View.

Use the following procedure to select the custom fields you want to add to the cube as dimensions. This action adds the data for the selected custom fields to the cube as dimensions when the cube is built.

Log on to Project Web Access as an administrator.

1. Click Server Settings.
2. In the Cube section, click Configuration.
3. On the Cube Configuration page, in the Cube Dimensions section, go to the Cube list and select the cube for which you want to specify dimensions.
4. The Available fields list is populated with custom fields for the cube you have selected.
5. Select the custom fields you want to add as dimensions, and then click Add to move them to the Selected dimensions list.