@mariahneu this is already part of SmartSheet.
When you create a new column, you can choose the "Text/Number" style. Also, if you "Edit Column Properties" by right-clicking on the column header (or double-clicking the column header), you should be able to select "Text/Number" if you already chose something else.
https://us.v-cdn.net/6031209/uploads/FEQZO55SDBCE/image.png
As a test I created a "test" column in my sheet with the "Text/Number" style. Even if I enter a pure number but with leading zeros, the zeros are not removed.