By default, to determine if a column should be read in as numeric, JMP reads down a column testing that values in a column are either numeric of character. Only if no non numeric values are found, does JMP read in the column as numeric. However, this is not really a problem. Once the column is in JMP, it is easy to convert the column to numeric. Just go to the Column Info screen and change the data type and modeling type to Numeric, Continuous. JMP will then convert all rows to numeric, setting all rows that are not numeric values to a missing value.
What method are you using to import the data?
Have you go through the document "Discovering JMP" under the Help pull down menu?
What version of JMP are you using?
Could you attach a sample Excel file to aid in solving your issue?
Jim