To add custom data you need to create file "convert_data_custom.json" in the application folder
This custom file have higher priority than default so you can change and fix default values

In this JSON-file create two dicts: "blocks" and "datas"
In block "blocks" you should write pairs "MC text id": "VS text id"

If your block have some blockstates:
Example: MC "oak_slab" has blockstate "type" ("bottom"/"top"/"double")
VS id can be "plankslab-oak-down-free" or "plankslab-oak-up-free"
Against of it, you can write "plankslab-oak-TYPE-free" and create smth like alias for blockstate "type"
Put into dict "datas" these:
"type": {
"bottom": "down",
"top": "up",
"double": "*"
}
Very important! Alias must be same as MC blockstate
In the VS id it must be in upper case

If you put symbol "*" in your VS id, these block will not be converted

You can find example in archive. It replaces every MC stone block to VS oak planks and reverses directions of blocks with state "facing"

You will also find a copy of the default conversion file there for review