Using MASM I can’t see any way to define the table as compactly as FASM can, but I can see a simpler way to define it.
;===================================================================================
include \masm32\include\masm32rt.inc
;===================================================================================
.data
Rot13Buf4 LABEL BYTE
DB 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26
DB 27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50
DB 51,52,53,54,55,56,57,58,59,60,61,62,63,64
DB "NOPQRSTUVWXYZABCDEFGHIJKLM"
DB 91,92,93,94,95,96
DB "nopqrstuvwxyzabcdefghijklm"
DB 123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142
DB 143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162
DB 163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182
DB 183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202
DB 203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222
DB 223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242
DB 243,244,245,246,247,248,249,250,251,252,253,254,255
ROT13BUF LABEL BYTE
N=0
REPEAT 65
DB N
N=N+1
ENDM
DB "NOPQRSTUVWXYZABCDEFGHIJKLM",91,92,93,94,95,96,"nopqrstuvwxyzabcdefghijklm"
N=123
REPEAT 133
DB N
N=N+1
ENDM
.code
;===================================================================================
start:
;===================================================================================
mov esi, OFFSET Rot13Buf4
mov edi, OFFSET ROT13BUF
xor ebx, ebx
REPEAT 256
movzx edx, BYTE PTR [esi+ebx]
printf("%d\t%d\t",ebx,edx)
movzx edx, BYTE PTR [edi+ebx]
printf("%d\n",edx)
inc ebx
ENDM
inkey
exit
;===================================================================================
end start