I'm certainly no expert, but won't you have to dig deep in the records for each of the boats? Uboat.net says that there were eight XB boats (U-116, U-117, U-118, U-119, U-219, U-220, U-233, and U-234) but they were originally designed as minelayers; of these, some were modified for use as "transport boats". It appears that any refueling of operational boats by XB's was almost as a sideline or an afterthought. After all, there were a number of instances where an early-returning Typ IX boat was ordered to provide fuel to a Typ VII boat (or boats). In fact, there were some Typ VII boats that passed fuel to others. Any refueling by a Typ XB would be sort of "lost in the background noise".

(BTW, I checked the list of non-commissioned boats. There are no uncompleted XB boats shown; there are two non-commissioned boats with no type listed but they are U-796 and U-798 -- all other boats in the 792-799 series (allocated) were Typ XVII "Walther boat" research vessels. So, it appears that the only boats planned to be built at XB boats were the 8 listed and all were indeed completed and commissioned.)

POST SCRIPT -- (goes to show what I know -- i.e. not much)

U-117 --
On 6 August U-66 met U-117 under Neumann and received perhaps 10 cubic meters of fuel and provisions. The executive officer of U-117, Frercks, and the doctor came aboard U-66. The supplying of oil had to be interrupted when a carrier- based (USS Card) plane attacked and sank U-117.

U-118,
late Sep 1942 1st Patrol
U-118 reached her operations area about 600 miles southwest of the Azores, at approximate position 300 14’ N. – 340 00’ W., without further incident. In the course of the next 10 days, she supplied eight U-boats, 500-tonners and 750-tonners, which were said to have been operating in the Caribbean area. At one period, prisoners stated, there were five boats waiting in the vicinity of U-118 for supplies. Three of these were serviced simultaneously as the other two kept look-out. While one was being refueled in tow, provisions were transferred to two others in small boats.
2nd Patrol (Nov., 1942)
She arrived at her operational area, again southwest of the Azores, about 27 November. In the course of the next ten days, she oiled and provisioned eight or nine U-boats, without unusual incident.
3rd patrol (following minelaying)
U-118 arrived at her operations area southwest of the Azores about 11 February (1943), remaining there until 21 February. In the course of that week she supplied seven or eight U-boats. A survivor explained that their mission was to supply U-boats returning from, or leaving for, patrols.
4th Patrol
On 4 June, 1943, U-118 found U-Manseck in a damaged state. U-Schnoor arrived at the spot shortly thereafter. Four of Manseck’s wounded were in serious condition. U-118’s doctor was rowed over to attend the wounded.
At Czygan’s suggestion, the three U-boats moved away from the vicinity on a course of 900 as soon as Manseck’s wounded were cared for. The following morning, both U-Manseck and U-118 took on from 40 to 45 tons of oil from U-Schnoor. U-118 returned to her area of operations. Soon thereafter, U-118 was sunk (12 June, 1943, USS Bogue).

U-220, sunk while supplying U-256, 28 October, 1943

U-233, sunk on minelaying mission (never supplied other boats) near Halifax, 5 May, 1944.

U-234, surrendered to US forces, May 1945 (never supplied other boats).