what are the impacts if I convert one of the column to primary key or add a primary key to an existing table
Having primary key is absolute necessity for any table.

But obviously before you do that you need to see whether adding a primary key will affect the application itself. If written in ASP.Net it will require model update. Also without primary key entity model looks at it as a view, which is mostly used for displaying data. Adding primary key would cause a conflict and you would have to explicitly create a new view.

If you already have a clustered index on some column, adding a primary key on another column would simply create a primary key constraint. Where as if you do not have a clustered index (heap table), and create a primary key it will implicitly create a clustered index as well on that column.

Probably worst option would be creating a new table and migrating data, simply due to the fact that you would loose all the indexes(if any) were on the table,also if the table had some relationship with the other table you wouldnt be able to delete it. Or if explicitly some one before you added some security rights on the table, would be lost.

If possible try to add primary key on already created table, and existing column.