I'm here, after a long time, but with good news. First of all, thank you to @huaracheguarache because his tip helped to me to write properlly the ubus calls.
To begin, I want to clarify something, because always there are a confusion about 802.11 k/v/r. They are often being mixed as part of the same thing, like a bounding condition, and this is not true. Each standard can work independently, of course there are sinergy points between them, but you need to understan what is the scope of each one:
802.11v: BTM requests. This feature allows an AP to steer a STA to a desired BSS, or just to suggest to him to check if they have a better BSS to connect (in facts, this is always a suggestion because the "last word" is always from the client).
802.11k: This is meant for metrics exchange. The most common usage is the Beacon report metrics, in which one device asks to another for information/metrics regarding the beacons a device can monitor.
802.11r: aka "Fast-Roaming"(really this should be called "Fast-Steering"). Is a mechanism to allow fast associations when a STA leaves a BSS and associates to another of the same network.
Done my disclaimer, I want to explain how I was able to make BSS changes with 802.11v without having to configure anything else, that had nothing to do with this issue.
I explain from an example:
ubus call hostapd.bss0 bss_transition_request '{"addr":"24:46:c8:bb:08:5f","disassociation_imminent":false, "abridged":true, "validity_
period":20, "neighbors":["3460f9226df2ef1900008095090603029b00"]}'
In this case, I am moving the STA of MAC address "24:46:c8:bb:08:5f" to the BSS of BSSID "34:60:f9:22:6d:f2". That is all, when I used this command, the BTM request/response occured:
My only doubt left is how if formed that ID which should be used to indicate the target BSS. This is not the BSSID. Is the BSSID+something, that probably depends on the SSID and password (an encoding, a hash or a signature by using both cretentials). I remember something about some IDs which are mainly relevant in 802.11r (like mobility domain) but at the moment I don't figure which one is this ID.
So my workaround, to obtain this ID, is by using:
ubus call hostapd.bss1 rrm_nr_get_own
Which returns something like:
{
"value": [
"34:60:f9:22:6d:f2",
"Pablomagno",
"3460f9226df2ef1900008095090603029b00"
]
}
And you can notice inmediatelly that the trailer after the BSSID is the same at least to the whole WiFi network which shares the SSID and (maybe) the password.