In Excel, when you change a cell's formatting to text, it doesn't actually change the underlying field type, it only changes the on-screen presentation of that value. So Monarch still sees a numeric field.
Suppose that cell B2 has the value 4000 under the heading AcctNum. To convert the numeric value to a text value, type the formula =TEXT(B2,"0") into, say, cell A2. Type the heading TextAcctNum into cell A1. Copy this formula to all of your account number rows and save the file.
When you define the external lookup in Monarch, connect the TextAcctNum to the account number field in your lookup table that Monarch already recognizes as Character, and all should be fine.
Let us know if this is a practical solution for you, and if it does indeed resolve your problem.
Taking Kruncher's suggestion from the other direction, you can make the lookup work using a calculated field.
So if your lookup Excel field is really numeric but your master database field is alpha just add a calculated field to your model to make a numeric version of your alpha field.
VAL(Alpha_field) will do that.
Then when you define the link to the Excel lookup file make the link using the new numeric calculated field rather than the original alpha version of it. Should work fine even if you hide the field.
Remember to save the model and the project if it is something you repeat regularly!!
So two approaches to choose from. I would use whichever is the most convenient at the time.
PS. the STR() function would allow the opposite (numeric to alpha) conversion should it be required.