my_alphs <- c("X","Y","Z")

my_str <- "LA**"
# Note that the length of the string can be longer than 4 and
# the position of the asterisk can be anywhere with different length

LAXX
LAXY
LAXZ
LAYX
LAYY
LAYZ
LAZX
LAZY
LAZZ

library(dplyr)

expand.grid(my_alphs, my_alphs) %>%
  mutate(var = paste0(gsub('\**', "", my_str), Var1, Var2)) %>%
  select(var)

#var
#1 LAXX
#2 LAYX
#3 LAZX
#4 LAXY
#5 LAYY
#6 LAZY
#7 LAXZ
#8 LAYZ
#9 LAZZ

expand.grid(my_alphs, my_alphs) %>%
  mutate(var = paste0(gsub('\**', "", my_str), Var1, Var2)) %>%
  select(var) %>%
  pull

#[1] "LAXX" "LAYX" "LAZX" "LAXY" "LAYY" "LAZY" "LAXZ" "LAYZ"
#[9] "LAZZ"