You are going to have to replace the whole hydraulic brake system for straight bars. I did a little research on this before I bought my Civante as I will eventually have to go that route as well as I get older in the next 2-3 years. They do make Tiagra straight bar shifters though. I would prefer if the Civante had mounting points for Caliper brakes as I prefer them as I can work on them myself. I do understand why they went with disc brakes as they have greater stopping power. Cantilever brakes would have worked well and they have great stopping power.
If you browse around different bike forums, some say that you can use straight bar hydraulic levers with road hydraulic disc brakes but the modulation will be somewhat different. It may be worth a try and if it does not work just replace the whole hydraulic break system to match the straight bar levers. Talk to your local bike shop if you have a good one that knows what it is doing with hydraulic disc brakes.
There are some options that you can try. You can use a stem extender like this-
Amazon.com : Delta Cycle & Home Bike Bicycle Stem Raiser Pro Extender Head Up Handlebar Raiser, Black, 1 1/8 inch : Bike Stems And Parts : Sports & Outdoors
www.amazon.com
And add an adjustable stem or a shorter stem with a 30-40 degree rise. I did this to my road bike after I had a cervical spine fusion due to a herniated disc. Ritchey makes a fairly light weight adjustable stem.
This one has a shorter reach of 80mm and is more adjustable with higher angles. It may be all you need
Amazon.com : FOMTOR 31.8 Adjustable Stem 0~90 Degree 80mm MTB Adjustable Bike Stem for 31.8mm(1.25") Handlebar Mountain Bike, Road Bike, Fixed Gear Bicycle, BMX : Sports & Outdoors
www.amazon.com
You will have to replace cables depending upon how high you raise the bars. I hope this helps.
One other thing, if you try a riser stem you can get a new handlebar with a short reach and short drop. I am buying a short reach 70mm and short drop bar for my new Civante and have added a shorter stem with a 35 degree rise. This will bring the brake levers closer to you and allow you to be more upright if paired with a stem rise and or adjustable stem.
Check out the Ritchey WCS Streem III alloy road bar. It is an aero bar like what's on the Civante.
If you do not know what reach and drop are see the pictures below
Best of luck,
Larry