This can be done via BIOS mod to make it visible to you (Dump BIOS region with FPT, modify that, unlock BIOS Lock, reflash BIOS region with FPT), or it you can change the current default option via grub using setup_var

I checked, and xHCI handoff is enabled by default already, so you should be good to go

XHCI Legacy Support, VarStoreInfo (VarOffset/VarName): 0x1A, VarStore: 0x20, QuestionId: 0x388, Size: 1, Min: 0x0, Max 0x1, Step: 0x0 {05 91 6E 0E 6F 0E 88 03 20 00 1A 00 10 10 00 01 00}
One Of Option: Enabled, Value (8 bit): 0x1 (default) {09 07 E2 15 30 00 01}
One Of Option: Disabled, Value (8 bit): 0x0 {09 07 E3 15 00 00 00}

One Of: XHCI Hand-off, VarStoreInfo (VarOffset/VarName): 0x1B, VarStore: 0x20, QuestionId: 0x389, Size: 1, Min: 0x0, Max 0x1, Step: 0x0 {05 91 ED 15 EE 15 89 03 20 00 1B 00 10 10 00 01 00}
One Of Option: Enabled, Value (8 bit): 0x1 {09 07 E2 15 00 00 01}
One Of Option: Disabled, Value (8 bit): 0x0 {09 07 E3 15 00 00 00}
Default: DefaultId: 0x0, Value (8 bit): 0x1 {5B 06 00 00 00 01} << comment below
Default: DefaultId: 0x1, Value (8 bit): 0x1 {5B 06 01 00 00 01} << Both BIOS profiles are set to 0x1 = enabled

@Lost_N_BIOS I'm not sure if you'll appreciate me bringing up an old(er) thread, but I'm looking to do something described similarly to the issue here. That is, I would like to add the BIOS option "Manual" to "Overclocking Mode" in the cheaper B660, or the B660-A Strix Gaming WiFi D4 specifically, to potentially enable BLCK overclocking on these boards. I suspect the option to use BCLK for overclocking is artifically limited, and not necessarily bound to any hardware component like DDR4/DDR5 support, altough external voltage regulators could be a thing. The support seems a bit widespread and on only some medium-end B660 boards as well. Only very few specific ASUS B660 boards seem to support this feature. As far as I know most Z690 would ("Unlock BCLK OC" in "Tweakers Paradise"), but it's pointless as they are too expensive relative to the performance gain of BCLK overclocking with cheap CPU's.

These B660 mobos support this feature, according to Der8aur: YT: watch?v=DUzExonH5TM
Strix B660-G WiFi (mATX)
Strix B660-F Gaming WiFi (ATX)

Rumors were going around about the ASRock B660 Steel Legend supporting BCLK overclocking, but my own findings were that following boards >don't< support it in all available BIOS versions:
Strix B660-A Gaming WiFi D4
Prime B660-Plus D4
Prime B660M-K D4
ASRock B660M-HDV
*the Steel Legend D4 is not available yet

As you can see I'm aiming for the somewhat lower-end B660 DDR4 motherboards, as getting the "Manual" function, and all the corresponding BCLK overclocking features done on a board like those would be great value.

To come to my question, I've been looking at your suggestions: "(Dump BIOS region with FPT, modify that, unlock BIOS Lock, reflash BIOS region with FPT), or it you can change the current default option via grub using setup_var" - The first short tut "Dump BIOS etc." I would like to know more about, but can't seem to find it. If enabling the option via grub using setup_var would be feasible for normal functionality, I would also like to know how to do so, but can find little on it, or am plain searching wrong. Could you please illiuminate me on where to start looking?

I have used FPT, MMTool, UEFITool, AMIBCP5, and maybe a few more to some extent while replacing/updating CPU microcode, but I never got to unlock BIOS Setup Configuration parts that would probably be functional if available.