Just multiply the prices by 10 before calculating the bands, then divide by 10 before storing/using the results.

Code:
function run()
{
StartDate = 20170501;
EndDate = 20170502;
NumWFOCycles = 10;
DataSplit = 60; // activate WFO
NumCores = 4;
BarPeriod = 5;
set(BINARY);
Slippage = 0;
Spread = Commission = 0;
LifeTime = 1;

WinPayout = 75;
LossPayout = 0;

set(PARAMETERS);
int signal = optimize(20,5,30,1);
vars Close = series(priceClose());
var NbDevUp = optimize(2,1,4,0.5);
var NbDevDn = optimize(2,1,4,0.5);
BBands((series(10*priceClose())),signal,NbDevUp,NbDevDn,MAType_SMA);
vars upperBand = series(rRealUpperBand/10);
vars lowerBand = series(rRealLowerBand/10);

plot("Bollinger1",rRealUpperBand,BAND1,BLACK);
plot("Bollinger2",rRealLowerBand,BAND2,GREY);
set(PLOTNOW);

if (crossOver(Close, upperBand))
enterShort(); //close short trades and open a long position
if (crossUnder(Close, lowerBand))
enterLong();

}