You'd need to DIM an array to get the user variable name and then compare the user input verses that array for matches.
DIM UserVar(2) AS STRING
FOR I = 0 TO 2
INPUT "Enter a variable name=>"; UserVar(1)
NEXT
DO
'Let user input whatever they want
'Parse what they want and compare their variable names to those stored in the array
'Display results
LOOP 'Until the user is finished
So at start the user can enter X, Y, Z for three variable names. Then the user can do things like X = 3. Y = 5. Z = X + Y... as long as you parse and code to allow the user such actions in your program.
** Fellippe beat me to it. I type too slow on my iPad. :'( I want my new PC already!