You can use DataFrame.from_dictwith DataFrame.insertif you want to select the position of a new column:
d = {'01/24/2017 01:10:23.1230':('a',12),'12/25/2016 10:12:45.128':('b',23),'11/16/2016 09:39:55.459':('c',45),'01/12/2017 15:55:20.783':('d',34)}
df = pd.DataFrame.from_dict(d, orient='index').reset_index()
df.columns = ['Date','value1','value2']
df.insert(0, 'userid', 123)
print (df)
userid Date value1 value2
0 123 01/24/2017 01:10:23.1230 a 12
1 123 12/25/2016 10:12:45.128 b 23
2 123 01/12/2017 15:55:20.783 d 34
3 123 11/16/2016 09:39:55.459 c 45
If a new column at the end is required DataFrame:
df['userid'] = 123
print (df)
Date value1 value2 userid
0 01/24/2017 01:10:23.1230 a 12 123
1 12/25/2016 10:12:45.128 b 23 123
2 01/12/2017 15:55:20.783 d 34 123
3 11/16/2016 09:39:55.459 c 45 123
Or a solution with assign:
df = df.assign(userid=123)
print (df)
Date value1 value2 userid
0 01/24/2017 01:10:23.1230 a 12 123
1 12/25/2016 10:12:45.128 b 23 123
2 01/12/2017 15:55:20.783 d 34 123
3 11/16/2016 09:39:55.459 c 45 123
EDIT by comments:
dict comprehension, 123:
d1 = {k:(123, v[0], v[1]) for k,v in d.items()}
print (d1)
{'01/24/2017 01:10:23.1230': (123, 'a', 12),
'11/16/2016 09:39:55.459': (123, 'c', 45),
'01/12/2017 15:55:20.783': (123, 'd', 34),
'12/25/2016 10:12:45.128': (123, 'b', 23)}
df = pd.DataFrame.from_dict(d1, orient='index').reset_index()
df.columns = ['Date','userid','value1','value2']
print (df)
Date userid value1 value2
0 01/24/2017 01:10:23.1230 123 a 12
1 11/16/2016 09:39:55.459 123 c 45
2 01/12/2017 15:55:20.783 123 d 34
3 12/25/2016 10:12:45.128 123 b 23