Dimensions with number values are exported to Excel as numbers, and their SeekTable formatting ("Format" specifier) is converted to Excel format equivalent if present.
You can set desired "Format" either in the cube configuration (for all reports) or for the concrete report (use 'gear' icon next to "Columns" or "Rows"). In particular, to force 2 digits after the decimal point you may specify {0:0.00} or, if you want a thousands separator, {0:#,##0.00} :
In SeekTable values are formatted with standard
.NET String.Format (+ some custom format specifiers), so all format specifiers that may be used with String.Format can be used in SeekTable.