Sadly there's no such feature in radare2 yet.
Remember, though, that at the end these variables are data that you can manipulate by manually editing them. Thus, you can modify their value (with some restrictions of course) to meet your needs.
For example, let's see the result of
afvd in some random function of a binary:
var local_10h = 0x7ffffa0c1870 0x00007ffffa0c1960 `....... r13 stack R W 0x1 --> rdi
We have only one variable,
local_10h which is located at
rbp - 0x10:
[0x00400637]> afvd local_10h
pxr $w @rbp-0x10
Now, let's modify
rbp-0x10 to be "ABCD":
[0x00400637]> wx 41424344 @ rbp-0x10
var local_10h = 0x7ffffa0c1870 0x00007fff44434241 `ABCD... r13 stack R W 0x1 --> rdi
So this is a way to manipulate a value of a variable.
radare2 is an Open-Source project with great community and developers, feel free to open an issue, or even better, propose a pull-request so we all can benefit from this feature.