Hi drrichard,
Spreadsheets can not "push" a value into a cell. They can only "pull" values from elsewhere.
Duplicate your original table (Table 1) and rename it Table 2. That will be your results table.
Table 2 will "pull" values from either the Lookup table (with RANDBETWEEN) if a cell in Table 1 is blank, or will "pull" the existing letter from Table 1 if it is not blank.

Formula in Cell A2 of Table 2 is
IF(ISBLANK(Table 1::A2),XLOOKUP(RANDBETWEEN(1,26),Lookup::$B,Lookup::$A),Table 1::A2)
Fill down and fill right.
In English, if the corresponding cell in Table 1 is blank, then:
- use XLOOKUP to search for a random number between 1 and 26 in Column A and return the letter in Column B
- else return the existing letter from Table 1
Regards,
Ian.