Drive XTAL input with logic signal
The question is now almost three weeks old, but I think I can provide some useful information. The BNO055 XIN32 can probably be driven by a logic signal, with details discussed below.

When this was originally posted, I had asked Bosch if the BNO055 XIN32 could be driven by a clock signal using VDDIO voltage levels. Bosch answered that yes, the XIN32 input can be driven that way, with XOUT32 left floating. I have edited this answer correspondingly.

The remainder of this answer analyzes the situation, and the system implications.

Bosch offers the BMF055, a 'Custom programmable 9-axis motion sensor', in the same package as the BNO055. The BMF055 has the same sensor characteristics (compare pages 2 and 3 of the two datasheets), but is programmable. I make the assumption that the BNO055 is a BMF055 with sensor fusion programming factory-supplied by Bosch. The BMF055 datasheet provides more information.

Section 9.1 of the BMF055 datasheet states that the embedded processor is an Atmel (now Microchip) SAM D20. The pin description table further shows specifically that pin 26 (BNO055 XOUT32) is SAM D20 'PA01', also 'optional 32kHz crystal output', and pin 25 (BNO055 XIN32) is SAM D20 'PA00', also 'optional 32kHz crystal input'. This seems pretty clear, the BNO055 presumably has an embedded SAM D20 connected to XIN32 and XOUT32.

The Atmel SAM D20 datasheet (http://www.atmel.com/images/atmel-42129-sam-d20_datasheet.pdf) provides details regarding XIN32. Conveniently, Bosch has used the Atmel SAM D20 nomenclature, you will find details of XIN32 and XOUT32 in the Atmel SAM D20 datasheet.

There are some system issues to deal with.

First, as to voltage levels, the SAM D20 datasheet section 8.1 shows that the external oscillator pins are in the VDDANA (analog) voltage domain, not the VDDIO (digital) voltage domain. The SAM D20 datasheet section 8.2.1 states that these voltages must be the same, unlike some other microcontrollers.

The Atmel SAM D20 supports an external clock connected to XIN32 (datasheet section 17.2). There is more detail in section 17.6.3: the SAM D20 has separate modes for XIN32 as a clock input and XIN32/XOUT32 supporting an external crystal. Presumably the Bosch firmware in the BNO055 sets the mode to external crystal rather than external clock. This has consequences:


The SAM D20 datasheet section 17.6.3 explicitly states that the 32kHz external oscillator (XOSC32K) external output (enabled by 'EN32K') is usable only when an external crystal is supplied, not when an external clock is supplied. The SAM D20 datasheet does not describe this external output in more detail, so I am not certain what this external output is, and how it would be affected by connecting an external clock while the oscillator mode is set to crystal.
The SAM D20 will drive the XOUT32 pin, because it expects an external crystal. This may or may not be a problem for you. The SAM D20 has an automatic gain control for XOUT32/XIN32, hopefully this will not cause any problems. Presumably the clock input levels will drive the AGC to its lowest gain.
The SAM D20 has a programmable startup delay for the 32.768kHz crystal. The SAM D20 datasheet table 40-25 specifies a maximum startup time of 30k cycles (slightly less than 1 second), so presumably Bosch selected a startup delay time on the order of 1 second. Your clock source must be active within the startup delay after the BNO055 is powered on.
Notice that a reset signal is not enough to reset the SAM D20 clock system, a power-on reset is required (SAM D20 datasheet section 14.8). So, if your 32kHz clock source can be reset during operation, you may have to control the power to the BNO055 in order to force it through a power-on reset when the clock source resets.


For the PCB I am designing, I decided to leave the BNO055 with its own 32.768kHz crystal instead of driving XIN32 with the 32.768kHz signal already available on the board.

I hope this is helpful.