My RPG experience is likewise very old, but one of the COBOL conversion techniques I know very well should be easily adaptable.
Decimal digits without a sign are called "Binary Coded Decimal" (BCD) and a typical usage for a six-digit BCD value is to hold a Gregorian Date with a two-digit year: MMDDYY.
So, I'll use MMDDYY to explain the technique:
(1) Move the 3-byte BCD field (right justified) to a 4-byte binary field.
==> MMDDYY becomes xxMMDDYY.
(2) Multiply by 16
==> xxMMDDYY becomes xMMDDYY0.
(3) Add 15
==> xMMDDYY0 becomes xMMDDYYF.
(4) Declare the 4-byte binary field as 4-byte packed decimal (7-digits). Does RPG have a "redefines" feature, like COBOL? If so, use it.
(5) Move the 4-byte packed decimal field to a 6-digit zoned-decimal (USAGE DISPLAY) field.
==> xMMDDYYF becomes .M.M.D.D.Y.Y ("." characters represent zone value of "F".