IMHO, I think you can do this.

Being an I2C master is easy to do in software.
All PICs to date bit-bang I2C master behaviour.
Even the PICs with I2C hardware.

Being an I2C slave is not so easy to do in software.
You'd need to spend all your time watching the I2C signals.
Or have the low-going edge of the SCL signal interrupting
the CPU so the CPU can hold SCL low to 'wait-state' the bus.

For this reason, if you want a PIC to be an I2C slave,
the I2C hardware helps a lot. Because bit-banging the
master behaviour is so easy, it is wiser just to implement
the I2C slave behaviour in hardware. Which is exactly what
Microchip have done. This has annoyed quite a few people who
feel this is like being told a micro has a UART and then
finding out this is receive-only and you have to bit-bang
serial transmission. In practice, it's not such a big deal
because I2C master transmissions are much faster and less
time-critical than RS232.

Where Microchip refer to "I2C interface hardware", substitute
the text ""I2C slave interface hardware". They really should
be more explicit about this.

Beware of the following!
This will save you days or weeks, of frustration.

1: Hardware

If you read Microchip's description of the SSP,
there is no way to get it to work!
See my previous postings about the SSP as an I2C slave.

2: Software

If you read Microchip's application note (AN557
I2C multimaster environment), the code does not work!
It does not correctly handle wait states when
reading data from a slave. When it tries to let SCL
go high, the slave is entitled to hold SCL low to
wait-state the I2C master (i.e. the PIC).
When their code sees this happen, it (wrongly) responds
by writing an error code to a PIC data register.

Microchip know about point 1,
I can forgive them making mistakes.
I can't forgive them not telling customers and
their own tech support engineers about them.
I had to chase that problem all the way to
a chap at their fabrication plant!

You have been warned!
Be ever alert for Microchip errors.

BTW, what kind of instruments do you make?