2.5ns analogue beamformer (8 to 1 channel)
Are the delays set or variable?
If they are set then you can use analog delay lines and add them together with a resistor network. Here are some examples http://micro.apitech.com/delay_lines.aspx, the switched coaxial delay lines are something that you might even be able to build yourself.

If the sensing arrangement would work and still be linear with the sensors in different positions you may be able to simply change the position of the sensors to introduce delay.

Otherwise you will need some very advanced equipment, something that can sample eight 10MHz signal at say 40MHz at even 8-bit accuracy will generate a data stream of 2.5Gb/s. You would need to program an FPGA unless you can find a bucket brigade type delay chip that can operate at several times your maximum frequency, if it exists it would be insanely expensive.

For combining the signals the easiest way is something like this:


Search op-amp adder for more details.