akshay@Aix:~/Desktop$ cat f1
BP         CM
752566 rs3094315
752721 rs3131972
753541 rs2073813
760300 rs11564776
12222222

akshay@Aix:~/Desktop$ cat f2
BP         CM
740857 1.984065
750235 2.009238
752566 2.012958
753269 2.013806
753541 2.014133
754745 2.014728
765948 2.019205
767038 2.019743
767070 2.019759
768448 2.020467
769551 2.020969
771521 2.021869

awk '

function find(Arr,key,arg,   low,upp,i){
			      low = upp = ""
			
			      for(i in Arr){
				            if(i<key)
						 {
						   low =  (low && low > i) ? low : i
						 }
					    if(i>key)
						 {   
					           upp =  (upp && upp < i) ? upp : i
						 }  	 
					     }
			    
			 low  =  (low != "") ? low : "-"
		         upp  =  (upp != "") ? upp : "-"
			 
			if(arg=="up"){return upp}
		   else if(arg=="dw"){return low}	 
}

function lin(x1,x3,y1,y3, x2){
				return (y1+(x2-x1)*(y3-y1)/(x3-x1))
			     }

 FNR==NR{ 
		A[$1] = $2
                next
        }
        {
		if($1 in A)
		{	
			# Exact match
			print $1,A[$1]
		} 
		else
		{
			# No match will interpolate
			x1=find(A,$1,"dw")
			x3=find(A,$1,"up")

		   # Whether we have upper and lower limit ?
		   if(x1 !="-" && x3 !="-")
			{
				# Yes interpolate
				y1=A[x1]
                                y3=A[x3] 
				print $1,lin(x1,x3,y1,y3, $1)
			}
			else
			{
				# No cannot interpolate so NaN
				print $1,"NaN"
			}
		}
	}
  ' f2 f1

akshay@Aix:~/Desktop$ ./interpolate 
BP CM
752566 2.012958
752721 2.01314
753541 2.014133
760300 2.01695
12222222 NaN